Various methods have been proposed for the preparation of alkanediol bis(aminobenzoate) such as 1,3-propanediol bis(p-aminobenzoate), including:
(1) a method in which nitrobenzoic acid chloride is reacted with an alkanediol in a pyridine solvent to prepare alkanediol bis(nitrobenzoate), and the alkanediol bis(nitrobenzoate) thus prepared is then reduced with hydrazine in the presence of a palladium or charcoal catalyst in an ethanol solvent (Japanese Patent Publication No. 10030/79 (corresponding to U.S. Pat. No. 3,932,360));
(2) a method in which nitrobenzoic acid and alkanediol are reacted in an anisole solvent to prepare alkanediol bis(nitrobenzoate), and the alkanediol bis(nitrobenzoate) is hydrogenated in the presence of a palladium-carbon catalyst (DE-OS No. 2,853,175 (corresponding to U.S. Pat. No. 4,283,549)); and
(3) a method in which nitrobenzoic acid and alkanediol are reacted in the presence of a p-toluenesulfonic acid catalyst without the use of a solvent to prepare alkanediol bis(nitrobenzoate), and the thus-formed alkanediol bis(nitrobenzoate) is hydrogenated in a methanol solvent in the presence of nickel and sodium dihydrogenphosphate (European Patent Application Laid-Open No. 13,956).
The method (1), however, has disadvantages in that it involves two steps of esterification and reduction, the yield of the desired alkanediol bis(aminobenzoate) is as low as 63% (esterification yield: 84%; reduction yield: 75%), and in that use is made of hydrazine which requires strict care in the handling thereof. Therefore, this method is not suitable for use in the commercial preparation of alkanediol bis(aminobenzoate).
The methods (2) and (3) have been developed in order to overcome the problems of the method (1). These methods, however, also suffer from disadvantages in that they involve two steps of esterification and reduction, the esterification and reduction reactions should be made at high temperatures for long periods of time--e.g., at 180.degree.-190.degree. C. for 6 hours and 160.degree. C. for 4 hours (under reduced pressure for 1 hour), respectively--in order to increase the esterification yield, and in that complicated operations are needed in the handling of hydrogen (since the reduction is achieved by hydrogenation), the recovery of the catalyst, and so forth. Thus, none of the methods are satisfactory as an industrial method of production of alkanediol bis(p-aminobenzoate).
In addition, there may be considered another method of production of alkanediol bis(aminobenzoate) which comprises reacting aminobenzic acid or its metal salt with dihalogenated alkane. In accordance with this method, however, since aminobenzoic acid or its metal salt contains an amino group and a carboxyl group or carboxylate group (-COOM) as active groups which react with the dihalogenated alkane, side reactions such as N-alkylation as well as the main reaction of esterification occur, producing various by-products. It is therefore impossible to prepare the desired alkanediol bis(aminobenzoate) in high purity and high yield.
In view of the various disadvantages of any one of the methods described above, there is clearly a need to develop a new method for preparing alkanediol bis(aminobenzoate).